The Lives and Opinions of Eminent Philosophers by Diogenes Laërtius (best free ebook reader txt) 📕

or more propositions; such for instance as the following: “If Dion walks, then Dion is in motion.” Those are conclusive which infer their conclusion specially, and not syllogistically; such for instance, as this: “The proposition it is both day and night is false. Now it is day; therefore, it is not night.”

Those again are unsyllogistic arguments which have an air of probability about them, and a resemblance to syllogistic ones, but which still do not lead to the deduction of proper conclusions. As for instance, “If Dion is a horse, Dion is an animal; but Dion is not a horse, therefore, Dion is not an animal.”

Again, of arguments, some are true and some are false. Those are true which deduce a conclusion from true premises, as for instance “If virtue profits, then vice injures.” And those are false which have some falsehood in their premises, or which are inconclusive; as for instance “If it is day, it is light; but it is day, therefore Dion is alive.”

There are also arguments which are possible, and others which are impossible; some likewise which are necessary, and others which are not necessary. There are too, some which are not demonstrated from their not standing in need of demonstration, and these are laid down differently by different people; but Chrysippus enumerates five kinds, which serve as the foundation for every kind of argument; and which are assumed in conclusive arguments properly so called, and in syllogisms, and in modes.

The first kind that is not demonstrated, is that in which the whole argument consists of a conjunctive and an antecedent; and in which the first term repeats itself so as to form a sort of conjunctive proposition, and to bring forward as the conclusion the last term. As for instance “If the first be true, so is the second; but the first is true, therefore so is the second.” The second kind that is not demonstrated is that which, by means of the conjunctive and the opposite of the conclusion, has a conclusion opposite to the first premise. As for instance “If it be day, it is light; but it is night, therefore it is not day.” For here the assumption arises from the opposite of the conclusion, and the conclusion from the opposite of the first term. The third kind that is not demonstrative is that which, by a negative combination and by one of the terms in the proposition, produces the contradictory of the remainder; as for instance “Plato is not dead and alive at the same time, but Plato is dead; therefore Plato is not alive.” The fourth kind that is not demonstrative is that which, by means of a disjunctive and one of those terms which are in the disjunctive, has a conclusion opposite to what remains; as for instance “It is either the first or the second; but it is the first; therefore it is not the second.” The fifth kind that is not demonstrative is that in which the whole argument consists of a disjunctive proposition, and the opposite of one of the terms, and then one makes the conclusion identical with the remainder; as for instance “It is either day or night; but it is not night; therefore it is day.”

According to the Stoics, truth follows upon truth, as “It is light” follows upon “It is day.” And falsehood follows upon falsehood; as “If it is false that it is night, it is also false that it is dark.” Sometimes too, truth follows from falsehood; for instance, though it is false that “the earth flies,” it is true that “there is the earth.” But falsehood does never follow from truth; for, from the fact that “there is the earth,” it does not follow “that the earth flies.”

There are also some arguments which are perplexed, being veiled and escaping notice; or such as are called sorites, the horned one, or the nobody. That is a veiled argument88 which resembles the following one: “two are not a few, nor three, nor those, nor four, and so on to ten; but two are few; therefore, so are ten few.”

The nobody is a conjunctive argument, and one that consists of the indefinite and the definite, and which has a minor premise and a conclusion; as for instance “If anyone is here, he is not in Rhodes.”

Such then are the doctrines which the Stoics maintain on the subject of logic, in order as far as possible to establish their point that the logician is the only wise man. For they assert that all affairs are looked at by means of that speculation which proceeds by argument, including under this assertion both those that belong to natural and also those which belong to moral philosophy: for, say they, how else could one determine the exact value of nouns, or how else could one explain what laws are imposed upon such and such actions? Moreover, as there are two habits both incidental to virtue, the one considers what each existing thing is, and the other inquires what it is called. These then are the notions of the Stoics on the subject of logic.

The ethical part of philosophy they divide into the topic of inclination, the topic of good and bad, the topic of the passions, the topic of virtue, the topic of the chief good, and of primary estimation, and of actions; the topic of what things are becoming, and of exhortation and dissuasion. And this division is the one laid down by Chrysippus, and Archedemus, and Zeno of Tarsus, and Apollodorus, and Diogenes, and Antipater, and Posidonius. For Zeno of Citium, and Cleanthes, have, as being more ancient they were likely to, adopted a more simple method of treating these subjects. But these men divided logical and the natural philosophy.

They say that the first inclination which an animal has is to protect itself, as nature brings herself to take an interest in it from the beginning, as Chrysippus affirms in the